Emotional reactions to perceived and real romantic infidelity have been implicated as a leading cause of homicide in many human cultures. Jealousy also plays a role in domestic violence and family breakup. Current research on jealousy has centered on Darwinian analyses that view jealousy as a specific innate adaptation that takes different forms in men and women. According to this perspective, men are innately disposed to react with hostility to indications of sexual infidelity, while women are innately disposed to react most intensely to indications of emotional infidelity. The research proposed here will critically examine this theory and contrast it with alternative accounts emphasizing social cognition. Psychophysiological reactions to jealousy-eliciting situations will be examined in detail to elucidate sex differences and the nature of the emotion of jealousy itself. Whereas almost all research pertaining to jealousy and infidelity has focused on one aspect of mating psychology at a time, the proposed studies will explore the interrelation of jealousy with other individual differences, including sexual experience, relationship history and sociosexuality. The proposed work will also depart from the exclusive focus on hypothetical responses by collecting and analyzing a large group of subjects' descriptions of emotional reactions to actual incidents of infidelity in their lives. The sample will include a wide age range of adults of homosexual and heterosexual orientations, allowing us to test the divergent predictions of the two theories concerning jealousy in homosexual individuals.